OCTG such as tubing and casing used in the excavation of gas wells and oil wells are usually connected to each other by threaded joints. In the past, the depth of oil wells was generally 2,000-3,000 meters, but in deep oil wells such as recent offshore oil fields, the depth of oil wells reaches 8,000-10,000 meters. In the environment of use, threaded joints for connecting such OCTG are subjected to various forces, such as axial tensile forces caused by the weight of the OCTG and the threaded joints themselves, the combination of internal and external pressures, and geothermal heat. Accordingly, threaded joints used for OCTG need to be able to maintain airtightness without undergoing damage even in such an environment.
A typical threaded joint used for connecting OCTG has a pin-box structure with an externally threaded portion formed on the end portion of a steel pipe (pin) and an internally threaded portion formed on the inner surface of a coupling (box), which is a separate connecting member. An unthreaded metal-to-metal contact portion is formed at the tip of the externally threaded portion of the pin and correspondingly it is also formed at the base of the internally threaded portion of the box. One end of the steel pipe is inserted into the coupling, and the externally threaded portion of the pin and the internally threaded portion of the box are then fastened until the unthreaded metal-to-metal contact portions of these two members are allowed to contact each other, thereby forming a metal seal to guarantee airtightness.
During the process of lowering tubing or casing into a gas or oil well, due to various problems, there are cases in which it is necessary to loosen a threaded joint which has been once fastened to connect two pipes, to lift the pipes and the threaded joint out of the well, to refasten the pipes with the joint, and then relower them. API (American Petroleum Institute) requires a joint that airtightness be maintained without the occurrence of severe seizing referred to as galling even if fastening (makeup) and loosening (breakout) are repeated ten times for a joint for tubing or three times for a joint for casing.
At the time of fastening, in order to increase the resistance to galling and airtightness, a viscous liquid lubricant which contains heavy metal powders and which is referred to as “compound grease” has conventionally been applied to the contact surfaces (namely, the threaded portions and the unthreaded metal-to-metal contact portions) of a threaded joint. Such a compound grease is specified by API Bulletin 5A2.
In the past, it has been proposed to form one or more layers by surface treatment such as nitriding, various types of plating including zinc plating and dispersed plating, and phosphating on the contact surfaces of a threaded joint in order to increase the retention of a compound grease on the contact surfaces and hence improve sliding properties. However, as described below, the use of a compound grease poses the threat of harmful effects on the environment and humans.
Compound grease contains large amounts of powders of heavy metals such as zinc, lead, and copper. When fastening a threaded joint, grease which has been applied is washed off or overflows to the exterior surface, and there is the possibility of the grease causing harmful effects on the environment and especially on sea life, particularly from harmful heavy metals such as lead. In addition, the process of applying a compound grease worsens the working environment, and there is a concern of harmful effects on humans.
In recent years, as a result of the enactment in 1998 of the OSPAR Treaty (Oslo-Paris Treaty) pertaining to preventing ocean pollution in the northeast Atlantic, restrictions concerning the global environment are becoming more strict, and in some countries, the use of compound grease is already restricted. Accordingly, in the excavation of gas wells and oil wells, in order to avoid harmful effects on the environment and humans, there has come to be a demand for threaded joints which can exhibit excellent galling resistance without using compound grease.
Up to now, there have been some proposals of threaded joints which can be used for connection of OCTG in an unlubricated state without application of a compound grease.
For example, JP-A 08-233163, JP-A 08-233164, and JP-A 09-72467 disclose threaded joints having, on the contact surfaces of a threaded joint, a lower phosphate (chemical conversion) coating and an upper solid lubricating coating containing a solid lubricant selected from molybdenum disulfide (MoS2) and tungsten disulfide (WS2) in a resin. The contact surfaces may be subjected, prior to the formation of a phosphate coating, to treatment for increasing the surface roughness or to nitriding treatment.
WO 2004/033951 discloses a threaded joint having a lower layer of a corrosion protective coating and an upper layer of a solid lubricating coating on the contact surfaces of the joint. The corrosion protective coating contains zinc powder in an epoxy resin, and the solid lubricating coating contains molybdenum disulfide (MoS2) or other solid lubricant in an inorganic binder.
However, in each of the above-described threaded joints designed for use in an unlubricated state in the prior art, the solid lubricating coating which is the outermost layer is a coating containing solid lubricant particles in a resin, which, as described below, causes some problems in its actual use.
OCTG are commonly transported by ocean shipping and stored outdoors. In order to prevent corrosion during shipment and storage prior to use, a rust preventive oil (or other liquid designed for rust prevention) is usually applied to the inner and outer surfaces of the pipe. In addition, in order to protect the thread surfaces and the unthreaded metal-to-metal contact portions during shipment and storage, a protector is often mounted on a threaded joint to protect each exposed contact surface of the pin and box of the joint. When a steel pipe for OCTG is shipped in a state in which a coupling is connected to one end of the pipe as shown in FIG. 1, protectors are mounted on the other end of the pipe and on the other end of the coupling.
Even if protectors are installed in this manner, the rust preventive oil which is applied to the inner and outer surfaces of the steel pipe prior to shipment penetrates into the inside of the protector during transport or storage. In addition, the inner and outer surfaces of the steel pipe become wetted by water supplied from condensation of moisture or rainfall during transport and storage, and this water also penetrates in the inside of the protectors. Both the rust preventive oil and water which have penetrated into the inside of the protector come to contact the solid lubricating coating formed as the outermost layer on the contact surfaces of the threaded joint. If installation of a protector is not carried out, such contact occurs more readily.
A solid lubricating coating is formed by particles of a solid lubricant such as molybdenum disulfide or tungsten disulfide dispersed in a binder, so the coating is inherently porous.
Therefore, if a rust preventive oil contacts a solid lubricating coating, it easily permeates into this coating which is porous. As a result, the solid lubricating coating cannot exhibit its function adequately, and there is the possibility of the galling resistance of the threaded joint markedly decreasing. It is conjectured that this is due to a decrease in lubricating performance due to a chemical reaction between the rust preventive oil and the solid lubricant or the binder, or due to an extreme pressure being generated in the rust preventive oil which is confined in the lubricating coating by the pressure which is generated at the time of fastening of a threaded joint, thereby resulting in the breakdown of the bonding of the lubricating coating.
Similarly, the condensed water and rainwater which penetrate into the inside of the protector and come to contact the solid lubricating coating easily permeate into this coating. As a result, there is the possibility of the lubricating properties of the coating decreasing due to a reaction of water with the solid lubricant or of the surface appearance being worsened particularly when the coating contains copper.
These problems caused by a rust preventive oil or water result from the fact that the outermost porous solid lubricating coating is not effectively protected. A corrosion protective coating formed underneath the solid lubricating coating for protection of the steel pipe itself as disclosed in WO 2004/033951 cannot solve these problems.